Multi level multileaf collimators

ABSTRACT

A multilevel MLC includes a first set and a second set of a plurality of pairs of beam blocking leaves arranged adjacent one another. Leaves of each pair in the first set are disposed in an opposed relationship and longitudinally movable relative to each other in a first direction. Leaves of each pair in the second set are disposed in an opposed relationship and longitudinally movable relative to each other in a second direction generally parallel to the first direction. The first and second sets of pairs of leaves are disposed in different planes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/861,368 filed Aug. 23, 2010, entitled “Multi Level MultileafCollimators,” the disclosures of all of which are incorporated herein byreference.

BACKGROUND

This invention relates generally to radiation apparatuses and methods,and in particular to multileaf collimators and methods of adjustingradiation beams useful in radiotherapy and other industries.

Multileaf collimators (MLCs) are widely used in radiotherapy machines tosupport various treatments including intensity-modulated radiationtherapy (IMRT) and arc therapy, etc. Conventional multileaf collimatorsinclude a single level of a plurality of beam blocking leaves arrangedin two opposing banks or arrays. Each leaf in a bank is longitudinallymovable relative to a leaf in the opposing bank. In operation each ofthe individual leaves is positioned to block a portion of a radiationbeam passing through the volume occupied by the leaf. The combinedpositioning of all leaves defines one or many apertures through whichthe unblocked radiation beam passes, and the aperture(s) define(s) theshape of the radiation beam directed to a treatment field at anisocenter.

To mitigate radiation leakage in single level MLCs, various leaf designsare developed including “tongue in groove” designs in which steps, wavesor similar geometries are provided on the leaf sides so that leafmaterials mutually overlap between leaves as viewed from a radiationsource. While a tongue in groove design may reduce leakage between leafsides, it unfortunately leads to undesirable underdose effects when MLCtreatment fields are combined. Some conventional MLCs are used incombination with one or two pairs of collimation jaws to reduce leakagebetween abutted leaf ends. One issue associated with the combination ofa MLC with collimation jaws is the increased bulk of a radiation systemand the resulting reduced clearance between the patient and movingequipment.

It is desirable to provide MLCs that can shape beams with highresolution so that the shaped beam conforms to a target volume as closeas possible. In general a MLC would provide for higher beam shapingresolution if the beam blocking leaves could be thinner. However,reducing the width of leaves to improve MLC resolution has limitationsand imposes challenges to MLC construction and operation. For MLCs usingscrew leaf drive systems for example, long slender drive screws may besusceptible to column buckling in a way that scales dramatically worsewith smaller screw diameters. Motors with a smaller diameter may also berequired.

SUMMARY

This invention provides for multi level MLCs and methods of shapingbeams that can significantly reduce various leakage effects and improvebeam shaping resolution. In some embodiments, a multilevel MLC comprisesa first set and a second set of a plurality of pairs of beam blockingleaves arranged adjacent to one another. Leaves of each pair in thefirst set are disposed in an opposed relationship and longitudinallymovable relative to each other in a first direction. Leaves of each pairin the second set are disposed in an opposed relationship andlongitudinally movable relative to each other in a second directiongenerally parallel to the first direction. The first and second sets ofpairs of leaves are disposed in different planes.

In some embodiments, each of the first and second sets includes a firstsection of a plurality of pairs of leaves having a first cross sectionand a second section of a plurality of pairs of leaves having a secondcross section different from the first cross section. In someembodiments, the first cross section of the leaves in the first sectionof the first set is different from the first cross section of the leavesin the first section of the second set.

In some embodiments, the leaves in the first and second setssubstantially focus on a single converging point. The leaves may have atrapezoidal cross section and generally flat side surfaces. Each leaf inthe first set may be offset from a leaf in the second set in a directiongenerally traverse to the first and second directions. The leaves in thefirst and second sets may be supported by one or more movable carriages.

In some embodiments, each leaf in the first set is offset from a leaf inthe second set by substantially half the leaf in a direction generallytraverse to the first and second directions. The leaves in the first setmay have a substantially same first cross section and the leaves in thesecond set may have a substantially same second cross section.

In one aspect a method of shaping radiation beams using a multi levelMLC is provided. The multi level MLC comprises first and second sets ofa plurality of beam blocking leaves disposed in first and second planes.Leaves in each of the first and second sets are arranged in two opposingarrays forming a plurality of pairs of leaves in the first and secondsets respectively. Leaves of each pair are arranged in an opposedrelationship and longitudinally movable relative each other, and thelongitudinal moving directions are substantially parallel generallytraverse to a beam direction. The leaves in the first and second setsare moved to block a selected portion of a radiation beam. In moving theleaves to produce treatment fields, generally, at least a portion of atleast one leaf in an array of the first set overlaps at least a portionof at least one leaf in an opposing array of the second set in the beamdirection. In some embodiments, the at least one leaf in the first setcan come in contact with a leaf in an opposing array in the first set.In some embodiments, the at least one leaf in the first set can come incontact with a leaf in an opposing array in the first set at a firstposition, and the at least one leaf in the second set can come incontact with a leaf in an opposing array of the second set at a secondposition that is offset from the first position in the leaf movingdirections.

BRIEF DESCRIPTION OF THE DRAWINGS

These and various other features and advantages will become betterunderstood upon reading of the following detailed description inconjunction with the accompanying drawings and the appended claimsprovided below, where:

FIG. 1 is a schematic diagram illustrating a radiation system thatincludes a multi level MLC in accordance with some embodiments of theinvention;

FIG. 2 is a cross-sectional view of an exemplary multi level MLC inaccordance with some embodiments of the invention;

FIG. 3 is an isometric view of an exemplary multi level MLC inaccordance with some embodiments of the invention;

FIG. 4 is a cross-sectional view of a portion of an exemplary multilevel MLC in accordance with some embodiments of the invention;

FIG. 5 is an isometric cut-away view of an exemplary multi level MLC inaccordance with some embodiments of the invention;

FIGS. 6A-6B are a cross-sectional view of a portion of an exemplarymulti level MLC in accordance with some embodiments;

FIG. 7A schematically illustrates an exemplary MLC leaf control methodin which abutted leaf ends at different levels close at positions offsetin the leaf moving directions;

FIG. 7B schematically illustrates an exemplary MLC leaf control methodin which abutted leaf ends at different levels do not physically toucheach other;

FIGS. 8A-8C illustrate a prior art beam shaping method;

FIGS. 9A-9C illustrate an exemplary beam shaping method in accordancewith some embodiments of the invention;

FIGS. 10A-10C illustrate an exemplary beam shaping method in accordancewith some other embodiments of the invention; and

FIG. 11 is a side cross-sectional view of an exemplary multi level MLCin accordance with some embodiment.

DETAILED DESCRIPTION

Various embodiments of multi level MLCs are described. It is to beunderstood that the invention is not limited to the particularembodiments described as such and may, of course, vary. An aspectdescribed in conjunction with a particular embodiment is not necessarilylimited to that embodiment and can be practiced in any otherembodiments. For instance, while various embodiments are described inconnection with X-ray radiotherapy machines, it will be appreciated thatthe invention can also be practiced in other electromagnetic apparatusesand modalities. It is also to be understood that the terminology usedherein is for the purpose of describing particular embodiments only, andis not intended to be limiting since the scope of the invention will bedefined by the appended claims, along with the full scope of equivalentsto which such claims are entitled. In addition, various embodiments aredescribed with reference to the figures. It should be noted that thefigures are not drawn to scale, and are only intended to facilitate thedescription of specific embodiments. They are not intended as anexhaustive description or as a limitation on the scope of the invention.

Various relative terms such as “upper,” “above,” “top,” “over,” “on,”“below,” “under,” “bottom,” “higher,” “lower” or similar terms may beused herein for convenience in describing relative positions,directions, or spatial relationships in conjunction with the drawings.For example, the term “level” or “upper or lower level” may be used forease of describing some embodiments when a radiation source is on thetop of an isocenter and a multi level MLC is positioned therebetween.The use of the relative terms should not be construed as to imply anecessary positioning, orientation, or direction of the structures orportions thereof in manufacturing or use, and to limit the scope of theinvention. As used in the description and appended claims, the singularforms of “a,” “an,” and “the” include plural references unless thecontext clearly dictates otherwise. For example, reference to “adirection” includes the opposite direction of the direction and aplurality of directions that are parallel to the direction. A directionincludes both linear and arc trajectories. As used herein the term“support body” may include a single support body member or a supportbody assembly comprised of a plurality of body members. The term “plane”as used in the plane of beam blocking leaves include both planar andcurved or cylindrical planes.

In general, the present invention provides a multi level MLC thatincludes two or more sets of beam blocking leaves in two or moredifferent levels or planes. The two or more sets of leaves may bearranged stacked one above the other and parallel so that all leaves maytravel in a substantially same direction. The two or more sets of leavesmay also be arranged offset such that each leaf in a set may be offsetfrom a leaf in a different set in a direction generally traverse to theleaf travel direction.

FIG. 1 is a simplified illustration of a radiation system 100 thatincludes an exemplary multi level MLC in accordance with someembodiments of the invention. The radiation system 100 includes aradiation source 102 that is configured to produce beams 103 such as ofphotons, electrons, protons, or other types of radiation. For example,in X-ray radiotherapy the radiation source 102 may include a targetwhich can produce X-ray radiation when impinged by energetic electronbeams. The radiation system 100 may include beam shaping components suchas a primary collimator 104 and optionally a secondary collimator 106 togenerally limit the extent of the beam as it travels away from theradiation source 102 toward an isocenter 108. A multi level MLC 110 canbe disposed between the radiation source 102 and the isocenter 108 tofurther adjust the shape and/or intensity of the beam 103 projectedtoward the isocenter 108. The MLC 110 and optionally a secondarycollimator 106 may rotate about an axis through the source 102 and theisocenter 108, facilitated by bearing 105. The radiation source 102,primary collimator 104, bearing 105, secondary collimator 106, and MLC110 may be enclosed in or attached to a structure such as a gantry,which may rotate about an axis such as a horizontal axis 109 through theisocenter 108. Thus in some embodiments, the radiation system 100 candeliver a treatment beam to a target in the isocenter plane 108 fromvarious angles, and the shape and/or intensity of the beam can bedynamically adjusted by the MLC 110 as the beam angle is swept orstepped around the target.

The radiation system 100 may also include various other components whichare not shown in FIG. 1 in order to simplify the description of theinvention. For example, the radiation system 100 may include aflattening filter for providing uniform dose distribution, an ionchamber for monitoring the parameters of a beam, and a field lightsystem for simulation of a treatment field, etc. The radiation system100 may also optionally include one or two pairs of collimation jawsmovable in x- and/or y-directions (lower jaws, upper jaws) to providefor rectangular shaping of beams. In some embodiments, the radiationsystem 100 may include one of the collimation jaw pairs in conjunctionwith a multi level MLC of the invention. In some embodiments, theradiation system 100 does not require collimation jaws; the inclusion ofa multi level MLC of the invention may effectively replace both theupper and lower jaws. As will be described in greater detail below, thedesign and control of the multi level MLC of the invention cansignificantly reduce various leakage effects, thus additionalcollimation jaws would not be required. Replacement of conventionalcollimation jaws would be an advantage as it reduces the bulk of aradiation system and improves the clearance between the patient andmoving equipment.

FIG. 2 is a cross-sectional view of an exemplary multi level MLC 210 inaccordance with some embodiments. To simplify description, two sets ofbeam blocking leaves at two different levels or planes are shown in FIG.2. It will be appreciated that three or more sets of leaves can bearranged at three or more different levels. As shown, the two or moresets 220, 230 can be arranged stacked and parallel. In each set, aplurality of leaves may be arranged in two banks or arrays forming aplurality of pairs of opposing leaves. Each leaf of a pair in a bank canbe longitudinally movable relative to the other leaf of the pair in theopposing bank. In some embodiments, the two or more sets 220, 230 can bearranged such that the leaves at different levels may travel in a samedirection. For example, the two or more sets 220, 230 may be arrangedsuch that all the leaves in the MLC 210 can travel in e.g. thex-direction generally traverse to the beam direction when in use.

The leaves of the MLC can be supported by a support body 212 which mayinclude such as frames, boxes, carriages or other support structures. Insome embodiments all the MLC leaves in different sets 220, 230 can besupported by a single carriage (unicarriage). The single carriage,supporting all the MLC leaves, can be driven such as by a poweredactuating mechanism in the MLC leaf travel direction. In someembodiments, the MLC 210 may include two carriages each supports aportion of the MLC leaves or each supports a level of leaves. FIG. 3illustrates an exemplary two level MLC 310 including two carriages 312,314. One carriage 312 may support half the MLC leaves on a same side ofall levels, and the other carriage 314 supports the other half on theopposing side. The two carriages 312, 314, each supporting half the MLCleaves, can be independently moved by powered actuating mechanism 330along the MLC leaf travel direction. The carriages 312, 314 may travelon guide rails 316. Numerous arrangements and types of guide rails andpowered actuators could be used to support and move carriages. The useof one or more carriages may provide advantages in that individualleaves and their travel can be shorter, and therefore have bettertolerance control, less cost, less weight, and can fit in a smallercover or similar structures. Combined speed of leaves and carriages canbe a treatment planning advantage. In some embodiments, the multi levelMLC of the invention does not require a movable carriage or carriages(carriageless).

As shown in FIG. 3, each of the MLC leaves 318 can be independentlymoved by an associated drive motor 320. The drive motors 320 can besecured to the support body such as a carriage or carriages 312, 314 andare coupled to position feedback devices, a computer and motion control(not shown). In operation the drive motors 320 receive signals from thecomputer and motion control and move to position individual leaves 318relative to the beam direction based on a treatment plan. Thepositioning of a leaf operates to block or adjust the radiation beamwhich is passing through the volume occupied by the leaf. The combinedpositioning of all leaves may define one or more aperture(s) throughwhich an unblocked radiation beam passes, and the aperture(s) may definethe shape of the radiation beam projected to a target which may belocated in the isocenter plane.

Returning to FIG. 1, the shape of the radiation beam 103 projected onthe isocenter plane 108 has a step or strip resolution at the beamboundary 112. The step resolution is a function of the width ofindividual leaves of the MLC 110 and the position of the leaves relativeto the isocenter 108 and the radiation source 102 from which the beam isemitted and diverged. In general, the step resolution would be higher ifthe leaves of the MLC 110 were thinner. Higher step resolution can alsobe provided by positioning the MLC 110 closer to the isocenter 108. Inthe description of the MLC definition and various radiation leakageeffects, the terms of “length,” “width,” “height,” “side,” and “end” ofa leaf may be used. The “length” of a leaf as used herein refers to theleaf dimension that is parallel to the leaf moving direction. The“width” of a leaf refers to the dimension of the leaf that is traversethe leaf moving direction and the direction of the radiation beam. The“height” of a leaf refers to the dimension of the leaf along the beamdirection. The “side” of a leaf refers to the surface adjacent toneighboring leaves in a bank. The “end” of a leaf refers to the surfaceof the leaf inserted into the field along the length.

FIG. 4 is a cross-sectional view of a portion of a multi level MLC 410showing some detail of the leaf arrangement in accordance with someembodiments. As shown, first and second sets of leaves 420, 430 may bearranged at two different levels. In some embodiments, the first andsecond sets 420, 430 can be disposed such that each leaf in a set (e.g.leaf 422 in the first set 420) can be offset from a leaf in another set(e.g. leaf 432 in the second set 430) along a lateral direction or adirection traverse to the leaf moving direction. For example in someembodiments, a leaf in the first set 420 or second set 430 can be offsetfrom a leaf in the second set 430 or first set 420 by substantially halfa leaf. Alternatively, in some embodiments the first and second sets420, 430 are disposed such that the gap between two adjacent leaves at alevel (e.g. the gap between leaves 422, 424 in the first set 420) ispositioned substantially at the middle of a leaf in another set (e.g.leaf 432 in the second set 430). The offset arrangement of leaves atdifferent levels provides for leaf projections that are also offset atthe isocenter. Therefore, in some embodiments the leaves in the firstand second sets 420, 430 are arranged offset each other to provide forprojections offset by approximately half a leaf width as projected atthe isocenter plane. This provides for substantially an equivalent ofdoubling MLC definition, or improving the step resolution to half ascompared to the definition of a single level MLC with leaves of the samephysical width. In some embodiments, the MLC may include three or moresets of leaves at three or more levels which may be arranged such thateach leaf at a level is offset e.g. by ⅓ or 1/n of leaf width asprojected at the isocenter where n is the number of sets of the MLC. Inembodiments with offset leaf arrangement, the number of leaves at alevel may be different from the number of leaves at another level. Forexample, in a two level MLC, a leaf array at the upper level may includeone more leaf than a leaf array at the lower level to ensure coverage byat least one single leaf at the sides of a symmetric MLC field.

The leaves in a set at a level may have a substantially samecross-section. For example, in some embodiments the leaves in a set mayhave a same trapezoidal cross-section. Other cross-sectional shapes ofleaves such as rectangular shape, tilted trapezoids, or trapezoids withstepped or wavy sides are possible. Alternating patterns ofcross-sections are also possible, such as trapezoid, rectangle,trapezoid, rectangle, and so on. The cross-sectional shapes describedherein do not refer to additional detail features in the cross-sectionthat provide support and guidance for the leaves, such as added hook ortab shapes. Due to divergence of radiation beam from a source, thephysical width of leaves at different levels may be different to providethe same projected width definition at the isocenter. For example, theleaves in a set closer to a source may have a narrower cross sectionthan that of the leaves in a set farther from the source. In someembodiments, the leaves in the first and second sets may be arrangedtilted or inclined to the source or substantially focus on a convergingvirtual point located substantially at the source. The focused leafarrangement may improve the quality of beam shaping at the isocenter.

The leaf side surfaces may be flat. In some embodiments, the adjacentleaf side surfaces may form a gap or spacing ranging from approximately10 to 100 micrometers to facilitate relative movement between theleaves. The leaf side gaps may be substantially the same at a level.Because leaves at a level may cover radiation leakage between leaf sides(gap leakage) at another level, the leaf sides of the multi level MLC ofthe invention require little or no “tongue in groove” design as inconventional MLCs. In some embodiments, the leaves may have atrapezoidal cross section and the leaves may be arranged such that theleaf side surfaces substantially focus on a converging virtual pointlocated substantially at the radiation source. This arrangement mayprovide the least leaf side penumbra. This arrangement can alsoeliminate or minimize “tongue and groove effect” because at a leaf levelthere is substantially no leaf material overlap between leaves as viewedfrom the radiation source. In a real situation where the radiationsource is of finite size, rather than a theoretical point, the radiationmay be thought to emanate from various “pixels” within that finitesource and, the leaf side surfaces may not be viewed as perfectlyfocused from every source pixel, and leaf overlap material at a leaflevel may contribute a slight tongue and groove effect from some ofthose pixels. Rather than an ideal focus on the source, a practicalcompromise such as a small step, wave, or a very slight defocused tiltmay prove to be a better balance between gap leakage and tongue andgroove effects.

The leaf ends can be round, flat, or in various other configurations.The penumbra of leaf ends closer to the source will tend to be greaterthan the penumbra of leaf ends farther from the source due to geometricprojection effects of the radiation source. For treatment planningpurposes, it would be desirable if leaf ends have approximately samepenumbra. For leaf ends that are substantially rounded, the otherwiseworse penumbra of the upper leaves can be partially mitigated by using alarger leaf end radius. A larger radius reduces the penumbra due totransmission through the leaf material (e.g. tungsten). A larger leafend radius may require taller leaves. Therefore, in some embodiment theheight of the upper level leaves is greater than the height of the lowerlevel leaves to insure approximately constant penumbra over the entireleaf travel range. In some embodiments the height of the upper levelleaves and the lower level leaves is substantially the same, but theupper level leaves have an end portion with a larger radius. FIG. 5shows with greater detail the end portion of the upper level leaves inan exemplary MLC 510. The MLC 510 includes upper level leaves 512 andlower level leaves 514 supported by a support body such as a leaf box516. The upper leaves 512 and lower level leaves 514 may have a mainportion of substantially same height (“H” as shown). The end portion 518of the upper level leaves 512 may have one or two “tooth” portions orprojections 520 a, 520 b extended e.g. either upward or downward, orextended both upward and downward to allow an increase of the leaf endradius of the upper leaves 512. The extended radius can mitigate thepenumbra of the upper leaves 512 without substantially increasing theweight or height of the upper level leaf bodies. Not increasing the leafbody weight beyond what is needed for shielding is desirable due topackaging volume and leaf weight constraints. The tooth extensions 520a, 520 b may be located outside of the leaf box 516 and use the spacenot otherwise needed. The resulting upper leaves 512 may have an endportion 518 with a “mushroom” shape in side view. If necessary tofurther mitigate unequal leaf end penumbra between the upper and lowerlevels, the radius of the lower leaf ends can be reduced below themaximum radius allowed by the leaf height.

The leaves may be constructed with various suitable radiationattenuating materials. To generally improve on leakage performance ofexisting beam limiting devices, the combined attenuation of all levelsof the MLC should be approximately 2.5 tenth value layers (“TVLs”) orgreater. Single leaves at one level should substantially mitigate thelocal leakage of leaf gaps at another level. In general, the leaf gapleakage that can be allowed at a level in the multi level MLC can begreater as compared to conventional single level MLCs since the leavesat another level can mitigate the gap leakage. Since small areas at theprescribed boundaries of treatment fields may be covered by only asingle leaf, the leaf height should be 1.5 TVLs or greater to performadequately.

In some embodiments, the multi level MLC of the invention can providefor a treatment field that is shaped by leaves all having the same widthdefinition at the isocenter. By way of example, a treatment field of40×40 cm² with a projected leaf width of ½ cm (¼ cm offset definition)can be provided using 322 individual leaves disposed at two levels. Asanother example, a treatment field of 40×40 cm² with a projected leafwidth of 1 cm (% cm offset definition) can be provided using 162individual leaves. It will be appreciated that treatment fields ofdifferent sizes with different width definitions can be provided by themulti level MLCs of the invention including different numbers ofindividual leaves based on specific applications.

In some embodiments, the multi level MLC of the invention may providefor a treatment field that is shaped by leaves of different widthdefinitions at the isocenter. The finer definition (e.g. ¼ cm) may beprovided in the central portion of the treatment field where precisionis more needed. This may reduce MLC cost and increase MLC reliabilitycompared to an MLC with a greater number of leaves allowing finedefinition throughout the entire treatment field. In an embodiment, thetransition of leaf width can be gradual. For example, the width ofleaves at a level can be progressively increased with distance from thecenter of the treatment field. Each leaf at a level may have aphysically different width dimension. Alternatively, each MLC level mayinclude leaf sections so that the transition of leaf widths is discreet.The transition can be made by placing transition leaves at specificlocations on one or both levels. The transition leaves insure that thegaps between leaves project at the desired spacing for the desireddefinition regions.

FIG. 6A is a cross-sectional view of a portion of an exemplary multilevel MLC 610 providing variable width definition in accordance withsome embodiments. The MLC 610 may include two or more sets of leaves620, 630 of different sizes which project different leaf widths at theisocenter (e.g., ½, 1, 2 cm etc.). To simplify description, leaves at alevel are shown as having a rectangular cross-section to betterillustrate the offset arrangement of leaves at different levels. Leavesmay have a cross-section of trapezoidal, rectangular, or other shapes.

At a first level 620, the MLC 610 may include a first section of leaves622 with a first cross-section that provides for a first substantiallysame width definition (e.g. ½ cm), a second section of leaves 624 with asecond cross-section that provides for a second substantially same widthdefinition (e.g. 1 cm), and optionally a third section of leaves 626with a third cross-section that provides for a third substantially samewidth definition (e.g. 2 cm) at the isocenter, and so on. At a secondlevel 630, the MLC 610 may include first, second, and optionally thirdsections of leaves 632, 634, 636 which may be arranged offset from thecorresponding first, second, and optionally third sections of leaves622, 624, 626 at the first level 620. The leaves of the first, second,and optionally third sections 632, 634, 636 at the second level 630 mayhave cross-sections that provide for width definitions at the isocentersubstantially same as the first, second, and optionally third widthdefinitions of the first level leaves 620 respectively. At one or bothlevel(s) such as the first level 620, one or more transition leaves 627may be disposed between the first and second sections 622, 624, oroptionally one or more transition leaves 628 between the second andthird sections 624, 626.

By way of example, the first sections of leaves 622, 632 at the firstand second levels 620, 630 may provide for ½ cm width definition or ¼ cmoffset definition at the isocenter, the second sections of leaves 624,634 at the first and second levels 620, 630 may provide for 1 cm widthdefinition or ½ cm offset definition, and optionally the third sectionsof leaves 626, 636 at the first and second levels 620, 630 may providefor 2 cm width definition or 1 cm offset definition. In someembodiments, a transition leaf 627 may provide for ¾ cm widthdefinition, or optionally a transition leaf 628 may provide for 1½ cmwidth definition. It should be noted that the above leaf widthdimensions are provided by way of example, and it will be appreciatedthat different width definitions may be provided for by a multi levelMLC 610 including leaves of different sizes. A multi level MLC withvariable width definitions allows the use of different types or sizes ofleaves in the MLC. For example, the MLC may include high definitionleaves in the middle section to define a treatment field closelyconformal to the target. In the outer section where high definition maynot be required, relatively low definition leaves may be used to reducemanufacturing cost and increase reliability of the MLC. By way ofexample, a multi level MLC with a variable leaf width configurationillustrated in FIG. 6A may provide a 40×40 cm² treatment field usingonly 162 leaves, which are far fewer than 322 leaves that would berequired for ¼ cm definition across the full field. FIG. 6B illustratesanother alternative embodiment with variable leaf widths which canprovide a 40×40 cm² treatment field also with 10 cm of ¼ cm definitionusing 202 leaves.

In some aspect the invention provides for a method of shaping radiationbeams. Using a multi level MLC and a control method provided by theinvention, various radiation leakage can be significantly reduced. Theleakage between leaf sides or gap leakage can be mitigated by using amulti level MLC with offset leaf arrangement (see FIGS. 3 and 6A-6B). Asdescribed above, a multi level MLC may include two or more sets ofleaves at different levels, and leaves at each level may be arranged intwo banks or arrays forming a plurality of pairs of opposing leaves ateach level. The two or more sets of leaves may be disposed generally inparallel so that all the leaves of the multi level MLC may travel in asubstantially same direction generally traverse to the beam direction.In a preferred embodiment, the two or more sets of leaves can bedisposed such that leaves at a level are offset from leaves at anotherlevel in a lateral direction (e.g., y-direction) generally traverse tothe leaf moving direction so that the leakage between leaf sides at alevel can be mitigated by leaves at another level.

To reduce leakage between abutted leaf ends that may be intended toclose in shaping a treatment field, the ends of the abutted leaves at alevel may close at a position slightly offset, in the leaf traveldirection (e.g., x-direction), from the position where the ends of theabutted leaves close at a different level. This would ensure that theabutted leaf end leakages are not superimposed but instead attenuated byat least a single leaf height. FIG. 7A schematically shows an exemplaryembodiment where abutted leaf ends close at positions 702, 704, whichare offset in the leaf travel direction (e.g. the x-direction) so thatthe rays through pairs of abutted ends are never superimposed. Inexecution, factors such as 3-dimensional effects including the presenceof separated treatment field regions the relative x and y positions oftheir field boundaries, and whether an even or odd number of fieldstrips separate regions etc. should be accounted for in determining theoffset positions. In general as shown in FIG. 7B, the abutted leaf endleakage can be mitigated if a portion of a leaf 712 in a leaf bank at alevel overlaps a portion of a leaf 714 in an opposing leaf bank at adifferent level as viewed from a radiation source. This would allowmitigation of abutted leaf end leakage to acceptable levels without everhaving to touch the opposing abutted leaves together. For example, aminimum physical gap of less than 1 mm between abutted leaf ends shouldsufficiently control leakage, yet still be manageable within controlaccuracies. Not requiring abutted leaf ends to ever actually touch canreduce control program and leaf drive mechanical complexity and increaseleaf drive reliability. Components such as springs and sacrificial“fuses” in a leaf drive nut as used in conventional MLCs to limitcollision damage can also be eliminated if abutted leaf contact is notneeded and such collisions become a rare event. Penumbra compromisesassociated with complex interlocking leaf end shapes can also beavoided. The ability to dynamically close leaf ends quickly, with lowleakage, even between momentarily separated field regions can be anadvantage to treatment planning. Such a creative offset control can beapplied to dynamically changing field regions. Dynamically separatingand recombining field regions can be created generally without evenmomentarily producing an unwanted region of high abutted leaf endleakage.

A multi level MLC and a method of shaping radiation beams have beendescribed. One of the advantages of the multi level MLC is that theoffset arrangement of leaves can effectively improve beam shapingresolution, and allow the same definition with leaves physically twiceas wide as for a single level MLC. The extra physical leaf width is aconsiderable construction advantage for achieving equal or higher MLCdefinition in a more limited volume, particularly for screw leaf drivesystems. For example, in screw leaf drive systems, long slender leafdrive screws may be susceptible to column buckling in a way that scalesdramatically worse with smaller screw diameters. Since the leaf drivescrew diameter is generally limited to not be greater than physical leafwidth, the invention greatly reduces the screw drive susceptibility tocolumn buckling by allowing leaf drive screws to be nearly doubled indiameter. In addition, wider leaves allow room for larger diametermotors. The general relaxation of leaf drive miniaturization can alsoallow more motor choices, faster leaf speeds, better manufacturingprocess control, higher performance margins, higher reliability, andeasier service access. These advantages are all desirable for dynamictreatments and MLC cost is also reduced.

Another advantage of the invention is that the use of the multi levelMLC can significantly improve the leakage effect over a single level MLCused in conjunction with one or two pairs of collimation jaws. FIGS.8A-8C and FIGS. 9A-9C compare a conventional beam shaping method withsome embodiments of the invention and their leakage effect. The graytones of the figure approximate the transmission of the radiation beampassing through the MLC similar to how it would appear on film, withmore radiation intensity being darker. To simplify calculations in thisexample, each level provides 2 Tenth Value Layer (TVL) attenuation. Thusthe transmission of radiation through a single leaf for this example isassumed to be 1% of the intensity of the original unattenuated radiationbeam. FIG. 8C shows intended radiation field regions shaped bycorresponding positions of a pair of collimation jaws (FIG. 8A) and asingle level MLC (FIG. 8B) acting in combination. FIG. 9C shows intendedradiation field regions shaped by a multi level MLC of the inventionincluding a first set (FIG. 9A) and a second set (FIG. 9B). In theconventional method, the combined leakage between the abutted leaf endsseparating field regions and the leakage between leaf sides are evidentas shown in FIG. 8C, whereas in the method using a multi level MLC ofthe invention, the combined leakage is significantly reduced as shown inFIG. 9C. Further, the combined leakage reduction of a conventional MLCshown in FIG. 8C is limited to a rectangle, while the combined leakagereduction of the multi level MLC of the invention extends nearly to theboundaries of the treatment field shown in FIG. 9C.

The leakage between abutted leaf ends can also be significantlymitigated using the control method described above. FIGS. 10A-10C showthat the abutted leaf leakage between rounded leaf ends at a singlelevel can be as much as 24% on the centerline. With the offset controlin the leaf travel-direction between levels, the abutted leaf endleakage can be reduced to less than 1%, as shown in FIG. 10C. Treatmentfield regions can be quickly separated and recombined without highleakage.

Because the multi level MLC and control method provided by the inventioncan effectively reduce leaf leakage to acceptable levels, collimationjaws such as y-direction jaws are not required to control leaf end toend leakage as it is in most conventional single level MLCs. A fairlysmall and lightweight y-direction jaw pair may optionally be used inconjunction with the multi level MLC to provide for continuousadjustability of field width. A y-direction jaw pair might also mitigatesmall points of leakage where the abutted leaf gap of one level alignswith a leaf side gap of the other level.

Shielding fixed to carriages or to a unicarriage may be used to provideadequate TVL coverage under all use cases. FIG. 11 shows a sidecross-sectional view of an exemplary two level MLC 1110 including upperlevel leaves 1112 and lower level leaves 1114 supported by a carriage1116. A small shielding block 1118 can be fixed to the top of thecarriage 1116 to insure adequate shielding under all use cases incombination with lowered leaf tail portions 1120, 1122.

The multi level MLC of the invention can be used in a radiotherapymachine to support various treatment options includingintensity-modulated radiation therapy (IMRT), arc therapy, and otherforms of radiotherapy. In intensity-modulated radiation therapy, themulti level MLC can be controlled to modulate the intensity and adjustthe shape of the beam conformal to the size, shape, and location of thetarget. In dynamic arc therapy, the radiation source may rotate indelivery of radiation from various angles. The multi level MLC can bedynamically controlled during rotation of the source to adjust the beamconformal to the size, shape, and location of the target from variousangles.

Those skilled in the art will appreciate that various othermodifications may be made within the spirit and scope of the invention.All these or other variations and modifications are contemplated by theinventors and within the scope of the invention.

What is claimed is:
 1. A multileaf collimator comprising: a first set ofa plurality of pairs of beam blocking leaves arranged adjacent oneanother, leaves of each pair in the first set being disposed in anopposed relationship and longitudinally movable relative to each otherin a first direction; and a second set of a plurality of pairs of beamblocking leaves arranged adjacent one another, leaves of each pair inthe second set being disposed in an opposed relationship andlongitudinally movable relative to each other in a second directiongenerally parallel to the first direction; wherein the first and secondsets of pairs of leaves are disposed in different planes and the firstset of pairs of leaves comprises a first quantity of pairs of leaves andthe second set of pairs of leaves comprises a second quantity of pairsof leaves wherein the first quantity and the second quantity aredifferent.
 2. The multileaf collimator of claim 1 wherein the firstquantity is greater than the second quantity by one pair.